1. Field of the Invention
The present invention relates to a method of connecting superconducting wires, each having a plurality of filament cores embedded in a stabilizing coating. More specifically, the present invention is concerned with a method for connecting the filament cores without impairing the high conductivity in the connected filament cores.
2. Description of the Prior Art
Superconducting wires are finding widespread use in various apparatus such as nuclear fusion experiment systems, nuclear magnetic resonance computers(NMR), tomographic devices(CT) and so forth. Generally, the length of superconducting wires is limited mainly for the reasons concerning the production thereof. In addition, superconducting machines are usually composed of a plurality of units which are fabricated separately and then assembled together. This essentially requires connection between superconducting wires of a predetermined length without impairing the superconductivity of such wires.
Hitherto, the connection between superconducting wires has been achieved by various methods such as brazing, press-bonding and welding. These conventional methods, however, inevitably increases the electric resistance at the connecting portions, resulting in an increased rate of heat generation and, hence, an increased consumption of liquid helium which is used as the cooling medium. This inconveniently makes it difficult to design and construct superconducting equipment having a large capacity.
In order to obviate this problem, a method is disclosed in Japanese Patent Application Laid-Open No. 16207/1984 for connecting superconducting wires. In this method, as shown in FIG. 23, the stabilizing coating of the superconducting wires 101, 102 to be connected to each other is removed to expose the super-conducting filaments 103 and 104, and the thus exposed superconducting filaments 103 and 104 are superposed on each other and placed in a connecting pipe 107. The connecting pipe 107 is then pressed so that the super-conducting filaments 103 and 104 are press-bonded to each other. According to this method, however, the superconducting filaments 101 and 102 are connected to each other only at their contacting outer surfaces, so that the connection between the superconducting wires cannot have a large critical current value.
Another problem encountered by this method is that, when the superconducting filaments 103 and 104 are pressed through the connecting pipe 107, the filaments in the vicinity of the region where both wires contact each other are bent inwardly substantially to the central region and rigidly held in this state. The thus bent filaments are subjected to tensile force with the greatest bending appearing at the portions of the filaments where they are exposed from the stabilizing coating. In consequence, the filaments tend to break due to a large degree of bending, and the characteristics of the superconducting wire tends to be deteriorated as a result of the work caused by the application of the pressing force. This in turn causes the critical current value at the juncture between the superconducting wires to fluctuate or to be decreased to a level which is much smaller than that offered by the elementary superconducting wire. The unfavourable effect such as breakage of the wires and deterioration in the characteristics due to excessive deformation caused by the press work can be reduced if the pressure applied to the connecting pipe 107 is decreased. In such a case, however, the closeness of the contact between the superconducting filaments 103 and 104 is impaired which causes an increase the contact resistance, resulting in an increased rate of heat generation.
The other methods are disclosed in Japanese patent application Laid-Open No. 165206/1981 and No. 60908/1984 for connecting superconducting wires. These methods, also, have posed some problems as well as those of the above method.
Thus, the conventional method for connecting superconducting wires requires delicate control or adjustment the step of exposing a surface of exposure of the superconducting conducting filaments to be connected, the step of overlapping the filaments, to be connected and the step of deforming the filaments caused by the press work.